This invention relates to an improvement in catalytic cracking whereby the cracking cycle life of the catalyst is extended for processes such as fluid catalytic cracking, and for catalytic dewaxing wherein a shape-selective catalyst is used.
The catalytic cracking of hydrocarbons, particularly petroleum fractions such as gas oils, to lower molecular weight gasoline and fuel oil products, is well known. This process is practiced industrially in a cycling mode wherein hydrocarbon feedstock is contacted with hot, active solid particulate catalyst without added hydrogen at rather low pressures of up to about 50 psig and at temperatures sufficient to support the desired cracking. As the hydrocarbon feed is cracked, "coke" is deposited on the catalyst particles and the catalyst loses activity and/or selectivity. The coked catalyst is disengaged from the hydrocarbon products, which are then separated into appropriate components. The coked catalyst particles, now cooled by the endothermic cracking and disengaged from the hydrocarbon products, are then contacted with an oxygen containing gas whereupon coke is burned off the particles to regenerate their catalytic activity and/or selectivity. During regeneration, the catalyst particles absorb the major portion of the heat generated by the combustion of coke with consequent increase of catalyst temperature. The heated, regenerated catalyst particles are then contacted with additional hydrocarbon feed and the cycle repeats itself.
Two major variants for the widely practiced cracking of gas oils are fluid catalytic cracking (FCC) and moving bed catalytic cracking, one version of which is known as Thermofor Catalytic Cracking (TCC). In both of these processes as commercially practiced, the feed hydrocarbon and the catalyst are passed through a "reactor"; products and recycle oil are disengaged; the catalyst is regenerated with cocurrent and/or countercurrent air; and the regenerated hot catalyst is contacted with more feed to start the cycle again. These two processes differ substantially in the size of the catalyst particles utilized in each, and also in the engineering of materials contact and transfer which is at least partially a function of the catalyst size. In fluid catalytic cracking (FCC), the catalyst is a fine powder of about 10 to 200 microns, preferably about 70 micron size. In the moving bed process the catalyst is in the shape of beads or pellets having an average particle size of about one-sixty-fourth to one-fourth inch, preferably about one-eight inch. However, the catalyst used in either variant comprise an acidic porous inorganic solid, such as silica-alumina, silica-magnesia, or the like; or an acidic form of a large pore zeolite, such as Zeolite X or Y, in a matrix.
It is important to note at this point that the cracking cycle life of the catalyst, (i.e. the elapsed time between regenerations) in either the FCC-type or the TCC-type operation is very short, regeneration by burning to restore activity and selectivity being required after a contacting period of from about 2 to about 30 minutes. This short cracking cycle life of the catalyst is characteristic of the non-hydrogenative cracking of gas oil feeds in the absence of added hydrogen.
Catalytic cracking has been successfully adapted in recent years to a shape-selective cracking process that has become known as catalytic dewaxing. The catalyst for this process contains, as active component, an intermediate pore size crystalline zeolite such as HZSM-5 which sorbs normal paraffins and certain methyl paraffins that are in the wax range, i.e. that are normally solids at room temperature, and cracks these waxy constituents to lower melting products. The catalytic dewaxing process is very effective for reducing the pour point not only of distillate fuel oils, but also for refined lubricant stocks including those made from vacuum tower distillate fractions or residuums as raw feeds. United States Reissue Patent No. 28,398 to Chen et al. describes and claims catalytic dewaxing, and is incorporated herein by reference as if fully set forth.
The catalytic dewaxing process per se is known and need not be described here in great detail. Its application for distillate fuels has been described, for example, by H. R. Ireland, et al. in "Hydrocarbon Processing", (under the title Distillate Dewaxing) May, 1979, Gulf Publishing Company, Houston, Tex., and is incorporated herein by reference for background purposes. Its application to lubricant manufacture is described, for example, in U.S. Pat. No. 4,259,170 to Graham et al., also incorporated herein by reference. The usual operation for catalytically dewaxing fuels or lubes employs a fixed bed of catalyst, and is normally started at as low a temperature (start-of-run temperature) as will provide a product meeting the target pour point specification. The catalyst activity usually declines relatively fast during the first days on stream, as made evident by an increase in pour point of the dewaxed raffinate, and the decline in activity is compensated by gradually increasing the dewaxing temperature. After some days, a second period may set in, sometimes referred to as the "line-out period", during which catalyst activity declines very slowly and requires a compensatory small increase in temperature, for example, 1.degree. F./day or less. When the temperature reaches a predetermined level (end-of-run temperature) above which the dewaxed product lacks stability or other prerequisites, the run is terminated to allow regeneration of the deactivated catalyst. The term "aging" will be used herein to refer to the catalyst deactivation which occurs either initially or during line-out operation. The term "catalyst cycle life" or "cycle life" as used herein means the length of time that elapses between the start-of-run temperature and the end-of-run temperature.
Although the cracking cycle life of a dewaxing catalyst is much longer than an FCC cracking catalyst, a number of waxy raffinates from different crudes that can be dewaxed by the solvent process are not now efficiently dewaxed catalytically because of a prohibitively short cycle life. Also, waxy raffinates that are now catalytically dewaxed would benefit from a slower rate of deactivation and extended cycle life.
The present invention is not directed to hydrocracking nor is it within the scope of this invention to use pressures of 1500 to 3000 psig which are characteristic for hydrocracking. Hydrocracking essentially involves hydrogenolysis and the hydrocarbon products formed are free of olefins and substantially richer in hydrogen content than the feed. In contrast, the cracking and dewaxing processes referred to herein produce some olefins, and the overall composition of the reactor effluent is substantially the same as that of the feed, i.e. the mol ratio of hydrogen to carbon is substantially unchanged.